Continuous-flow heater with a safety circuit

ABSTRACT

A continuous-flow heater for heating a liquid includes a fluid-accessible channel arrangement with at least one heating channel configured for heating the liquid with at least one electric heating element arranged therein, an electronic control system configured to control electronic circuit breakers in order to electrically operate the heating element and to control the heat output of the heating element, a flow measuring device electrically connected to the electronic control system and configured to detect the volume flow rate of the liquid flowing through the channel arrangement and to provide a pulsed measurement signal, and a safety circuit connected on the input side to the flow measuring device and on the output side to the electronic control system in order to provide a supply voltage for controlling the electronic circuit breakers depending on whether the volume flow rate is greater than a minimum volume flow rate specification.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Application No. EP19160042.8 filed Feb. 28, 2019, the entire content of which is incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a continuous-flow heater for heating a liquid, comprising a fluid-accessible channel arrangement with at least one heating channel, which is configured for heating the liquid with at least one electric heating element arranged therein; an electronic control system which is configured to control electronic circuit breakers in order to electrically operate the at least one heating element and to control the heat output of said heating element, the electronic control system being designed as a microprocessor controller; a flow measuring device, which is electrically connected to the electronic control system and configured to detect the volume flow rate of the liquid flowing through the channel arrangement and to provide a pulsed measurement signal, where the number of pulses of the measurement signal correlates to the magnitude of the volume flow rate.

BACKGROUND OF THE INVENTION

Such continuous-flow heaters are sufficiently well-known from the prior art. For example, document DE 10 2008 011 117 A1 shows a continuous-flow heater with a flow meter which is designed as a measuring turbine that has a pulse output. The measuring turbine is used to measure the volume flow rate or the flow rate of the fluid flowing through the heating device, the number of pulses per time unit being a function of the volume flow rate. The pulse signals are applied to a timing element which is formed by a microcontroller. If the flow rate of the fluid falls below a minimum volume, or if the flow rate signals of the flow meter are completely absent, this causes the timing element to trip. Furthermore, in this case the heating element of the continuous-flow heater is switched off to protect against overheating.

The disadvantage is that safety shutdown of the heating element is performed by the microcontroller of the control circuit itself. Any malfunctions of the microcontroller and/or the program executed with the microcontroller may mean that there is no guarantee that the heater will be shut down reliably in every case if the volume flow rate falls below a minimum. It is therefore the object of the present invention to propose a continuous-flow heater which guarantees reliable shutdown of the at least one heating element if the volume flow rate of the liquid to be heated is too low, even if the electronic control system for controlling the heat output is faulty or damaged.

SUMMARY OF THE INVENTION

The object is achieved by a continuous-flow heater with the features referred to hereinbefore in that the continuous-flow heater comprises a safety circuit which is connected on the input side to the flow measuring device and on the output side to the electronic control system, in order to provide a supply voltage for controlling the electronic circuit breakers depending on whether the volume flow rate detected by the flow measuring device is greater than a minimum volume flow rate specification. The safety circuit according to the invention has the advantage that, regardless of the electronic control system, in the event that the volume flow rate of the liquid flowing in the channel arrangement falls below the minimum volume flow rate specification, the heating of the at least one heating element is reliably shut down. This shutdown process takes place separately and completely independently of the electronic control system. Possible malfunctions of the electronic control system, particularly in the program sequence of the microprocessor controller, system crashes or “freezing” of the microprocessor, do not affect the proper shutdown of the heating of the at least one heating element by the safety circuit. Advantageously, the electronic control system is also designed and configured to electrically shut down the at least one heating element on detecting a drop below the minimum volume flow rate specification. This results in a high degree of redundancy. As long as the electronic control system is working properly, the at least one heating element can be shut down in the event of a lack of volume flow rate by the electronic control system which is designed as a microprocessor controller. In any case, shutdown of the at least one heating element in the event of a lack of volume flow rate is performed by the safety circuit according to the invention, which shuts down the at least one heating element under the conditions mentioned, irrespective of the functional capability of the electronic control system.

An expedient embodiment of the invention is characterised in that the safety circuit comprises at least one digital counter, the counter input of which is connected to the flow measuring device, such that the pulsed measurement signal is present at the counter input of the digital counter, at least one counter output of the digital counter being connected to the input of an electronic switching element, which enables or disables the supply voltage for controlling the circuit breakers according to the status of the counter output. Advantageously, in this way the supply voltage for controlling the circuit breakers is only enabled if a predetermined number of pulses from the flow measuring device has been processed via the counter input of the digital counter. This ensures that the heating elements are only heated if the flow rate measuring device has actually detected the presence of the volume flow rate in the channel arrangement. In particular, this safety function is also provided if the electronic control system is faulty or inoperative, for example, due to a faulty program sequence of the microprocessor controller.

A preferred development of the invention is characterised in that the digital counter has at least one counter disabling input and in that this counter disabling input is electrically connected to the at least one counter output. In other words, the digital counter is switched so that it goes into self-holding as soon as a counter value is reached, causing the relevant counter output to change from a low level to a high level. In this way, it is achieved—without there being any need for further components or elements—that the safety circuit is configured to be monostable, i.e. it automatically maintains the status of providing the supply voltage for controlling the electronic circuit breakers, as long as the volume flow rate detected by the flow measuring device is greater than the minimum volume flow rate specification. This state is only left again if the detected volume flow rate falls below this minimum volume flow rate specification or even drops completely to zero.

According to a further preferred embodiment of the invention, the digital counter is designed as a ring counter. This embodiment as a ring counter has the advantage that the counter outputs of the digital counter are not configured to be binary-coded, but rather every counter reading is linked to a change in the output level of one of the counter outputs. After counting a pulse, for example, the output zero is activated, while after counting n pulses every nth counter output is activated. Thus, by connecting one of the counter outputs to the counter disabling input, it is particularly easy to determine at what counter reading the digital counter circuit should go into self-holding. By selecting a different counter output, it is particularly easy to adjust which counter reading has to be reached before the safety circuit goes into the self-holding state for providing the supply voltage for controlling the electronic circuit breakers.

A further expedient embodiment of the invention is characterised in that the digital counter has a reset input, which is electrically connected to a digital output of a triggerably configured monostable multivibrator, the trigger input of the monostable multivibrator being configured to control whether or not the digital level at a digital output of the monostable multivibrator changes once, and the trigger input of the monostable multivibrator being electrically connected to the flow measuring device, such that the pulse measurement signal is present thereat. This has the effect that, if the pulses of the measurement signal are absent or in the event that the time lag between the pulses exceeds a predetermined time interval, the digital counter is reset and thus the supply voltage for controlling the electronic circuit breakers is switched off. Advantageously, the heating of the at least one heating element is thus switched off within a very short time if the flow measuring device does not detect any volume flow rate or detects one that is lower than the minimum volume flow rate specification. The monostable multivibrator is also referred to as a mon-stable trigger circuit.

According to a further preferred embodiment, the monostable multivibrator is configured in such a manner that the digital level of the digital output changes once as soon as the trigger input is free from pulses of the measurement signal of the flow measuring device. In other words, the digital level at the digital output of the monostable multivibrator only changes if the flow measuring device does not detect any flowing volume flow. For example, the monostable multivibrator is configured in such a manner that the digital output has a low level while pulses of the measurement signal are present at the trigger input, as a result of which the digital counter is in “counting mode”. If the pulses of the measurement signal are absent, the digital output of the monostable multivibrator assumes a high level and the digital counter is reset.

A further expedient embodiment of the invention is characterised in that the time constant of the monostable multivibrator is selected in such a manner that said time constant is a multiple of the maximum pulse length of the measurement signal in each case. This ensures that the digital level at the digital output of the monostable multivibrator does not change as long as pulses of the measurement signal of the flow measuring device are present at the trigger input. A level change at the digital output of the monostable multivibrator only takes place if no pulses of the measurement signal are received at the trigger input at least for the time determined by the time constant.

According to a further preferred embodiment of the invention, the safety circuit has a digital output which is configured to signal to the microprocessor controller whether or not the supply voltage for controlling the electronic circuit breakers is provided by the safety circuit. In this way, it is also additionally possible to use the microprocessor controller to process and evaluate the respective supply voltage status for controlling the electronic circuit breakers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an exemplary embodiment of the safety circuit according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Continuous-flow heaters for heating a liquid are already sufficiently known from the prior art. The drawing therefore does not include a separate illustration of such a continuous-flow heater. The continuous-flow heater comprises a fluid-accessible channel arrangement with at least one heating channel configured for heating the liquid. At least one electric heating element, preferably a bare wire heating element, is arranged in the heating channel.

The continuous-flow heater further comprises an electronic control system which is configured to control electronic circuit breakers, for example semiconductor switching elements, such as thyristors or triacs, in order to electrically operate the at least one heating element and to control the heat output of said heating element. The electronic control system is designed in particular as a microprocessor controller, thus having at least one microprocessor with the usual further electronic components which are regularly required for the operation of such a microprocessor.

The continuous-flow heater further comprises a flow measuring device, which is electrically connected to the electronic control system and configured to detect the volume flow rate of the liquid flowing through the channel arrangement and to provide a pulsed measurement signal, the number of pulses of the measurement signal correlating to the magnitude of the volume flow rate.

The flow measuring device is preferably designed as a flow turbine which is arranged in the flow path of the liquid either within the channel arrangements or in a liquid inflow or outflow route. The flow measuring device is designed in particular to provide a pulsed measurement signal. The number of pulses per time unit is a measure for the magnitude of the volume flow rate of the liquid flowing through the continuous-flow heater. If the volume flow rate is equal to zero, the measurement signal generated by the flow measuring device is pulse-free, i.e. is at low or high potential. The number of pulses of the measurement signal, when the volume flow rate present is not equal to zero, is preferably proportional to the magnitude of the volume flow rate.

The continuous-flow heater according to the invention comprises a safety circuit 10, a possible switching variation of which is shown by way of example in FIG. 1. On the input side, the safety circuit 10 is connected to the flow measuring device. Thus, the pulsed measurement signal is present on the measurement signal line 11. On the output side, the safety circuit 10 is connected to the electronic control system, not shown in the drawing, via a supply voltage line 12.

The safety circuit 10 according to the invention provides a supply voltage for controlling the electronic circuit breakers via the supply voltage line 12. Whether or not the supply voltage for controlling the electronic circuit breakers is provided via the supply voltage line 12 depends on whether or not the volume flow rate detected by the flow measuring device is greater than a minimum volume flow rate specification. The safety circuit 10 is thus configured in such a manner that it is only possible to control the electronic circuit breakers if it is determined that a minimum volume flow rate in accordance with the minimum volume flow rate specification is flowing through the fluid-accessible channel arrangement.

If the volume flow rate detected by the flow measuring device remains below this minimum volume flow rate specification, no supply voltage for controlling the electronic circuit breakers is provided via the supply voltage line 12, thus at any rate preventing heating of the at least one heating element when the volume flow rate is too low. The heating element or heating elements are also reliably switched off in any case by the safety circuit 10 as soon as the volume flow rate detected falls below the minimum volume flow rate specification. This switching off takes place independently of the microprocessor controller. The safety circuit according to the invention ensures that the heating elements are reliably switched off even if the microprocessor controller is faulty.

The safety circuit 10 according to the invention preferably has at least one digital counter 13. The counter input 14 of the digital counter 13 is electrically connected to the flow measuring device via the measurement signal line 11, such that the pulsed measurement signal is present at the counter input 14 of the digital counter 13. The digital counter 13 preferably has a plurality of counter outputs 15. As shown in FIG. 1, the digital counter 13 has, for example, nine of the counter outputs 15 which are designated as outputs Q0 to Q9 in the circuit diagram according to FIG. 1.

At least one of the counter outputs 15 is electrically connected to the input of an electronic switching element 16. The electronic switching element 16 is configured to enable or disable the supply voltage, which is present by way of example at pin 17 in FIG. 1, for controlling the electronic circuit breakers. By way of example, FIG. 1 shows how one of the counter outputs 15, namely Q3, is connected via the series circuit of the resistor 18 and the series resistor 19 to the base 20 of the switching element designed as NPN transistor 21. The presence of the supply voltage for controlling the electronic circuit breakers on the supply voltage line 12 may optionally be displayed via the light-emitting diode 22 with its series resistor 23.

The digital counter 13 preferably has a counter disabling input 24. The counter disabling input 24 of the digital counter 13 is configured to interrupt or continue the counting function of the digital counter 13 as a function of the signal level present at this counter disabling input. The counter disabling input 24 is electrically connected, as shown in FIG. 1, to one of the counter outputs 15 via the resistor 18 which may also be designed as a 0-ohm bridge.

If, due to the incoming pulses at counter input 14, the digital counter 13 reaches a counter value which results in Q3 of the counter outputs 15 changing from a low level to a high level, the counter disabling input 24 is also set to a high level and the counter value of the digital counter 13 is “frozen” as it were. In other words, the digital counter 13 is switched such that it goes into self-holding on reaching a certain counter value. The counter value at which this happens depends on which of the counter outputs 15 is fed back at the counter disabling input 24. The drawing shows the resistors or 0-ohm bridges 25, 26 for this which are optionally fitted instead of the resistor 18.

The safety circuit 10 further comprises a monostable multivibrator 27. The monostable multivibrator 27 is configured to be triggerable, this means that it is possible via a trigger input 28 to control whether or not the digital level at a digital output 29 of the monostable multivibrator 27 changes. The trigger input 28 of the monostable multivibrator 27 is electrically connected via the measurement signal line 11 of the flow measuring device such that the pulse measurement signal of the flow measuring device is present at a trigger input 28.

The digital output 29 of the monostable multivibrator 27 is connected to a reset input 30 of the digital counter 13. If no liquid flows through the fluid-accessible channel arrangement, the measurement signal provided by the flow measuring device on the measurement signal line 11 is pulse-free, and thus has a constant electrical level.

The time constant of the monostable multivibrator 27, that is the delay time which elapses until the digital level at the digital output 29 changes after a trigger process via the trigger input 28, is determined by the capacitance 31 and the size of the resistor 32. If, as previously described, a signal with constant level is present at the trigger input 28, the digital output 29 changes its digital level after the time constant set by the capacitance 31 and the resistor 32 at the latest.

These digital pulses reach the reset input 30 of the digital counter 13 via the line 33, such that if pulses of the measurement signal are absent via the measurement signal line 11, the counter value of the digital counter 13 is reset, all outputs Q0 to Q9 of the digital counter assume a low level and the supply voltage for controlling the electronic circuit breakers via the switching element 16 is switched off, so that the supply voltage line 12 is de-energised.

Further preferably, the monostable multivibrator 27 comprises the resistor 34 as well as the capacitor 35 which form a timing element via which the reset input 36 of the integrated switching circuit 37 is moved into a defined state during the switching on process of the safety circuit 10. The integrated circuit, for example, is a re-triggerable monostable multivibrator 27 with reset function, for example an integrated circuit of the type 74 HC123. Further preferably, an integrated circuit of the type 74 HC 4017 is used as the digital counter 13.

The previously described monostable multivibrator 27 is consequently configured in such a manner that the digital level at the digital output 29 changes if the trigger input 28 is free from pulses of the measurement signal of the flow measuring device.

Preferably, the time constant of the monostable multivibrator 27, which determines the cycle duration of the digital pulses, is selected in such a manner that said time constant is a multiple of the maximum pulse length of the measurement signal in each case. It is achieved by selecting the said time constant of the monostable multivibrator 27 so that a change of the digital level only takes place at the digital output 29 if no more pulses come from the flow measuring device via the measurement signal line 11 or, if the time gap between pulses has grown so large because of a low volume flow rate that said time gap is greater than the time constant defined by the capacitance 31 and the resistor 32. This ensures that the digital counter 13 is only reset if no volume flow rate or only an extremely low volume flow rate is determined by the flow measuring device. In both the latter two cases, the supply voltage for controlling the electronic circuit breakers is switched off in each case via the electronic switching element 16.

Optionally, a counter output line 38, which is electrically connected to one of the respective counter outputs 15, is routed out to a connection 41 via a resistor 39 with a downstream diode 40. The status of each counter output 15 can be queried as a status signal at the connection 41 by the electronic control system. The diode 40 prevents feedback via the electronic control system. 

1. A continuous-flow heater for heating a liquid, comprising a fluid-accessible channel arrangement with at least one heating channel, which is configured for heating the liquid with at least one electric heating element arranged therein; an electronic control system which is configured to control electronic circuit breakers in order to electrically operate the at least one heating element and to control a heat output of said heating element, the electronic control system being designed as a microprocessor controller; a flow measuring device, which is electrically connected to the electronic control system and configured to detect a volume flow rate of the liquid flowing through the channel arrangement and to provide a pulsed measurement signal, the number of pulses of the measurement signal correlating to a magnitude of the volume flow rate; and a safety circuit which is connected on an input side to the flow measuring device and on an output side to the electronic control system in order to provide a supply voltage for controlling the electronic circuit breakers depending on whether the volume flow rate detected by the flow measuring device is greater than a minimum volume flow rate specification.
 2. The continuous-flow heater according to claim 1, wherein the safety circuit comprises at least one digital counter having a counter input connected to the flow measuring device, such that the pulsed measurement signal is present at the counter input of the digital counter, wherein at least one counter output of the digital counter is connected to an input of an electronic switching element, which enables or disables the supply voltage for controlling the circuit breakers according to a status of the at least one counter output.
 3. The continuous-flow heater according to claim 2, wherein the digital counter has at least one counter disabling input electrically connected to the at least one counter output.
 4. The continuous-flow heater according to claim 2, wherein the digital counter is designed as a ring counter.
 5. The continuous-flow heater according to claim 2, wherein the digital counter has a reset input, which is electrically connected to a digital output of a triggerably configured monostable multivibrator, wherein a trigger input of the monostable multivibrator is configured to control whether or not a digital level at a digital output of the monostable multivibrator changes once, and wherein a trigger input of the monostable multivibrator is electrically connected to the flow measuring device, such that the pulse measurement signal is present thereat.
 6. The continuous-flow heater according to claim 5, wherein the monostable multivibrator is configured in such a manner that the digital level of the digital output changes once as soon as the trigger input is free from pulses of the measurement signal of the flow measuring device.
 7. The continuous-flow heater according to claim 5, wherein a time constant of the monostable multivibrator is selected in such a manner that said time constant is a multiple of the maximum pulse length of the measurement signal in each case.
 8. The continuous-flow heater according to claim 1, wherein the safety circuit has a digital output which is configured to signal to the microprocessor controller whether or not the supply voltage for controlling the electronic circuit breakers is provided by the safety circuit. 